A tube forming apparatus having a plurality of rotatable rolls is used to form a work of a plastic sheet-shaped material, such as sheet metal, into a tubular shape.
One known tube forming apparatus, as a first tube forming apparatus, includes a rotatably supported single main roll and a plurality of rotatably supported subsidiary rolls (see Patent Document 1 for example). This apparatus is designed to pass a work between the main roll and each of the subsidiary rolls sequentially to bend the work toward the main roll side, thereby forming the work into a tubular shape.
Another known tube forming apparatus, as a second tube forming apparatus, includes a hard main roll and an elastic roll which are designed to rotate by being pressed against each other (see Patent Document 2 for example). This apparatus is designed to pass a work through the nip defined between the two rolls to wrap the work around the peripheral surface of the hard roll, thereby forming the work into a tubular shape.